Ion transport apparatus and process

ABSTRACT

PCT No. PCT/NZ95/00060 Sec. 371 Date Jan. 6, 1997 Sec. 102(e) Date Jan. 6, 1997 PCT Filed Jul. 6, 1995 PCT Pub. No. WO96/01681 PCT Pub. Date Jan. 25, 1996The invention relates to a method and apparatus for inducing the mono-directional transport of ions across a conducting polymer membrane which separates electrolyte solutions, by creating a potential gradient across the conducting polymer membrane.

This application is a 371 continuation of PCT/NZ95/0006 filed on Jul. 6,1995.

TECHNICAL FIELD

The invention relates to the mono-directional transport of ions througha conducting polymer membrane and to an apparatus for inducing suchmono-directional transport.

BACKGROUND ART

Two approaches have been used to apply conducting polymers (CP's) toseparation technology. The first involves coating the CP film onto thestationary phase of an ion chromatography medium. Separation wasachieved by varying the redox state of the polymer during elution. Usingthis approach, Deinhammer et. al. (Anal. Chem. 1991, 63, 1884)successfully demonstrated the separation of the adenosine nucleotides,AMP and ATP. The second approach employs CP's as either free standing orcomposite membranes through which ion transport could be achieved.Transportation of ions was similarly induced by modifying the conductivestate of the polymer matrix.

Mirmohseni et. al. (J Intelligent Material Systems and Structures, 1993,4, 43) demonstrated the transport of Na⁺ and K⁺ ions through a freestanding membrane of polypyrolle (PPy). The anion employed as the dopantwithin the membrane was p-toluene sulphonate (PTS⁻). The experimentalsetup used comprised a two compartment cell separated by a PPy membrane.A current was applied potentiostatically by a three-electrode system inone of the compartments, using the membrane as the working electrode.Transportation of ions was achieved by oscillating the membrane betweenanodic and cathodic potentials. The result was the selective transportof K⁺ ions over Na⁺, the extent of which depended on the pulse widthemployed.

Although the process described by Mirmohseni et. al. achieved continuousion transport through the membrane, transport could not be maximised.The experimental setup dictated that ions absorbed selectively from onecompartment of the cell, were desorbed simultaneously into bothcompartments. A mono-directional flow of ions could not be achieved.

It is therefore an object of the invention to provide a means capable ofinducing mono-directional transport of ions through a conducting polymermembrane.

SUMMARY OF THE INVENTION

In broad terms the invention comprises a method of inducingmono-directional transport of ions between electrolyte solutions, themethod comprising separating the electrolyte solutions with a conductingpolymer membrane and creating a potential gradient across said membrane.

Preferably the potential gradient is created by using the conductingpolymer membrane as a shared working electrode.

In broad terms the invention further comprises an apparatus for inducingmono-directional transport of ions between electrolyte solutionsseparated by a conducting polymer membrane by applying a potentialgradient across the polymer membrane.

Preferably the potential gradient through the conducting polymermembrane is achieved by a three electrode system in each solution, thethree electrode systems comprising a reference electrode, a counterelectrode, and a shared working electrode and wherein the shared workingelectrode is the polymer membrane separating the solutions.

Preferably the conducting polymer membrane is free-standing.

Preferably the conducting polymer membrane is a composite membrane madeof a conducting polymer incorporated into either a non-conductive orconductive media.

Preferably the potential gradient through the conducting polymermembrane remains constant.

Preferably the conducting polymer membrane is selected from polypyrrole,polythiophene, polyacetylene, polyfuran, polyaniline or polyphenylene orderivatives of these.

Preferably the apparatus further includes combination pH probes.

Preferably the electrolyte solutions are constantly stirred.

Preferably the counter electrodes are stainless steel, titanium, gold,platinum or carbon counter electrodes.

Preferably the reference electrode is an Ag/AgCl or SCE electrode.

Preferably the potential gradient across the conducting polymer membraneis varied.

Preferably the potential gradient across the conducting polymer membraneis varied by holding one electrolyte solution at a constant potentialand allowing the potential in the other electrolyte solution to vary.

Preferably there are more than two electrolyte solutions each separatedby a conducting polymer membrane.

Preferably the potential gradient across the conducting polymer membraneis up to 2400 mV, preferably up to 2000 mV, and more preferably up to1400 mV.

Preferably the method and/or apparatus is used to remove ions from wine,geothermal bore water, wool scour or sea water.

Preferably the ions transported are cations and are selected from thegroup consisting of H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba and Ra.

Preferably the ions transported are anions and are selected from thegroup consisting of nitrates, sulphates, halides, phosphates, andperchlorates.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments are illustrated with reference to the accompanyingdrawings by way of example. In the drawings:

FIG. 1 shows the mechanisms for the reduction (a and b) and oxidation (cand d) of a polypyrrole film.

FIG. 2 shows the schematic diagram for the setup of an electrochemicalcell of the present invention.

FIG. 3 shows the cyclic voltammogram of a PPy membrane in NaCl.

FIG. 4 shows the concentration profile for the single ion transport ofNa⁺ through a PPy membrane.

FIG. 5 shows the concentration profile for the single ion transport ofK⁺ through a PPy membrane.

FIG. 6 shows the concentration profile for the transport of an equimolarmixture of K⁺ and Na⁺ through the membrane.

FIG. 7 shows the pH profiles during transport of an equimolar mixture ofK⁺ and Na⁺.

FIG. 8 shows the concentration profile for the transport of K⁺ ions withthe potentiostat operated in a master/slave mode.

FIG. 9 shows the transport of a number of mono-valent ions through a PPymembrane.

FIG. 10 shows the transport of K⁺ ions from wool scour through a 4 cm²PPy membrane.

DETAILED DESCRIPTION OF THE INVENTION

The setup preferably used to induce mono-directional transport of ionsbetween electrolyte solutions comprises a two compartment cell separatedby a conducting polymer membrane. The ions transported may be mono ordivalent cations or anions as is known in the art. For example typicalions transportable by the method or apparatus of the invention includeH, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra and the nitrates,sulphates, halides, phosphates and perchlorates. Of the halides, themost important are the chlorides, fluorides and iodides in commercialapplications although the remaining halides may also be transported.

Although the transport of anions was not demonstrated experimentally, itis known that they are expelled and incorporated into a conductingpolymer membrane in a similar fashion to that of cations. As portrayedin the mechanism (FIG. 1, a and c), if the anion is small and mobile,reduction of the membrane (a) leads to expulsion, whereas oxidation (c)leads to incorporation. Transport of anions across a conducting polymermembrane has been demonstrated by Mirmohseni, A Price, W E and Wallace GG (in print) for a number of sulphonated compounds with varyingmobilities.

Preferably the conducting polymers used in the membrane includepolypyrrole, polythiophene, polyacetylene, polyfuran, polyaniline andpolyphenylene or derivatives of these. However other conducting polymersmay also be used as will be known in the art. The membranes may bechemically or electrochemically grown and may be free-standing orcomposites when incorporated into either a non-conductive or conductivemedia as will again be known in the art.

While polypyrrole (PPy) membranes have generally been used in theexamples (see Examples 3-10 herein) any of the membranes referred topreviously could be used to equal effect as will be known in the art.All the membranes listed operate via the same conduction mechanism as isshown in FIG. 1 and would thus be viable options for use in the methodor apparatus of the present invention.

Ion transport through the membrane is induced by applying a potentialgradient across the conducting polymer matrix. Preferably potentials arecreated on either side of the membrane using three-electrode setups ineach compartment comprising reference electrode, counter electrode, andthe conducting polymer membrane as a common, or shared, workingelectrode. In this way potentials are applied simultaneously to theconducting polymer/electrolyte solution interfaces on either side of thepolymer membrane to create the potential gradient across the membrane.As will be apparent to a person skilled in the art the invention shouldnot be seen to be limited to a two compartment cell however.

Usually complementary potentials will be created on either side of themembrane. Such potentials may range between about +1200 mV and -1200 mVcreating a potential gradient of 2400 mV across the membrane. Whilecreating complementary potentials across the membrane (eg +400 mV/-400mV) will induce the transport of ions through the membrane, movement ofions will occur so long as a potential gradient is formed across themembrane. For example a -200 mV/+400 mV (600 mV gradient) combinationwill also induce mono-directional transport as will potential gradientssuch as 0 mV/±700 mV (700 mV gradient); -100 mV/+400 mV (500 mVgradient) or -200 mV/+700 mV (900 mV gradient). Creating such potentialswill therefore lead to the creation of an "anodic" and a "cathodic"compartment. The essential requirement is that a potential gradientacross the polymer membrane is created.

The potentials created on either side of the polymer membrane arepreferably held at a constant value and thus the potential gradientacross the membrane is also preferably constant. Howevermono-directional transport of ions will occur if one side of the cellsetup is allowed to fluctuate. In such a situation the other side of thesetup must however be held at a constant potential. In this way amaster/slave-type system (as is described in more detailhereinafter--see FIG. 8) is created which allows greater flexibility inthe use of the system.

Optimisation of the potential across the conducting polymer membrane tomaximise ion transport will depend on the particular application towhich the system has been applied, and on the electrolyte solutionsused. Such factors will be readily discernable to a person skilled inthe art and optimisation of transport will be a matter of testing only.

FIG. 1 summarises the results of reducing and oxidising the polymermembrane. Conventionally prepared polypyrrole (PPy) in its conductivestate contains positive charges dispersed throughout the matrix. Eachcharge is associated with a negatively charged dopant ion species, andcharge density is in the order of one ion pair to every 4 pyrrolemonomer units. Reduction of the polymer effectively neutralises thepositive charge within the matrix, and renders the polymernon-conductive. Neutrality of the dopant anion, A⁻, is achieved byeither desorption of the dopant into solution (1a), or absorption of acation into the polymer (1b). Conversely, oxidation of the reducedpolymer results in either absorption of anions into the matrix (1c) ordesorption of cations (1d) into solution, and reintroduces conductivityinto the matrix. The prevailing mechanism largely depends upon themobility of the dopant ion within the polymer. For example the use ofPTS⁻ (p-toluene sulphonate) by Mirmohseni et al (see prev) as the dopantion would predominantly use the mechanism as shown in FIG. 1b and FIG.1d. As shown by G G Wallace (Chem. in Britain 1993, 967) PTS⁻ has arestricted mobility within the PPy matrix. Thus the mechanism oftransport during the reduction and oxidation of the membrane would beadsorption (FIG. 1b) and absorption (FIG. 1d). It is this mobility thatdistinguishes the polymer as having either cation or anion-exchangeproperties.

Experimental

Chemicals

Pyrrole and p-toluenesulphonic acid sodium salt were obtained from theAldrich Chemical company. Pyrrole was distilled and stored undernitrogen prior to use. All other reagents were analytical grade.Distilled water was used throughout.

Instruments

A Perkin-Elmer 703 Atomic Absorption spectrometer was used for cationdetermination.

Cyclic voltammetry (CV) was carried out using a conventional threeelectrode electrochemical cell with a stainless steel (316) counterelectrode, a Ag/AgCl reference electrode and a free standing PPymembrane was the working electrode. The cell comprised one of thecompartments used for the transport experiments. Voltammograms wereperformed on a PAR 173 potentiostat with a 273 interface, and with theaid of the Headstart programme.

The transport experiments of FIGS. 3 to 7 were performed using identicalPAR 173 potentiostats.

Membrane preparation

Polypyrrole membranes (8 cm×8 cm) for use in all experiments wereprepared in aqueous solutions of pyrrole (0.1M) and p-toluenesulphonicacid sodium salt (0.1M) on stainless steel working electrodes.Electrochemical polymerisation was carried out galvanostatically usingan anodic current density of 2.8 mA cm⁻² for 20 minutes. The electrodeswere held in a vertical orientation in order to optimise coverage of theelectrode surface and minimise dendritic growth. The membranes werecarefully peeled from the electrode surface and washed thoroughly withdistilled water. Membranes were stored in distilled water untilrequired. The membrane thickness was approximately 70-80 micrometers.

Electrochemical Ion Transport Cell

The electrochemical cell used for transport experiments is depicted inFIG. 2. The cell consists of two externally clamped perspex compartments(A1 and A2), each with a capacity of 100 mL and separated by aconducting polymer membrane (M). Rubber gaskets (S1 and S2) were used toseal the cell from leaking. The area of the conducting membrane exposedto the electrolytic solutions contained in the compartments (A1 and A2)was 20 cm². Each compartment contained a stainless steel counter (C1 andC2) and a reference electrode (R1 and R2). The membrane (M) constitutedthe common working electrode. In addition, the compartments wereseparately stirred either mechanically or magnetically. In the mixed iontransport experiments the compartments (A1 and A2) contain combinationpH probes (pH1 and pH2). The electrodes in each compartment wereseparately connected to a PAR 173 potentiostat.

Transport Experiments--FIGS. 3 to 10

Transport was induced through the membrane by simultaneously applying apotential of -400 mV to one side of the cell (referred to as the`cathodic compartment`) and a potential of +400 mV to the other(referred to as the `anodic compartment`). As the two surfaces of themembrane were different in morphology, the membranes were placed withthe side peeled from the electrode facing the cathodic compartment ofthe cell.

Using the electrochemical ion transport cell described previously and asdepicted in FIG. 2, transport experiments were carried out on thechloride salts of the three cations Na⁺, K⁺ and Ca²⁺. Initialexperiments focused on the individual cations, later experiments lookedat the effect of transporting equimolar mixtures of (1) K⁺ and Na⁺, and(2) K⁺ and Ca²⁺. Only during the latter experiments was the pH of eachcompartment monitored. 0.1M solutions of the ions for transport wereplaced in the cathodic compartment of the cell, while the anodiccompartment contained an electrolytic solution with a different cation.Potentials were applied simultaneously, and the extent of ion transport,into the anodic compartment, determined by periodically removing samplesand analysing them by atomic absorption. A fresh membrane was used ineach transport system.

FIG. 3 depicts the cyclic voltammogram (CV) of a PPy membrane in a 0.1MNaCl solution vs an Ag/AgCl reference electrode. The electrolyticsolution was not deaerated prior to commencement of the CV. The scanrate=40 mV/sec. The forward sweep depicts oxidation of the film andcorresponds to ion expulsion. The reverse sweep depicts reduction of thefilm and ion inclusion. The CV indicates that the redox potentials arecentred around zero volts verses Ag/AgCl. This is similar to the resultobtained by Walton et. al. (Analyst, 1992, 117, 1305) for a PTS⁻ dopedPPy film in an aqueous solution of Et₄ NPTS. From the CV in FIG. 3 theoxidation/reduction potentials chosen for ion transport were +/-400 mV.

FIG. 4 shows the concentration profile, measured in the anodiccompartment, for the transport of Na⁺ through the membrane. Theconcentration of Na⁺ ions varies from 1×10⁻⁶ M at T=0 to approximately1.1×10⁻⁵ M at T=200 min. After an initial delay of a few minutes, twotransport rates were observed. An initial rapid rate followed by a laterslower rate. The initial delay is thought to correspond to the timetaken for the ion-depleted membrane to be filled-up with ions, a factorlargely dependent on the thickness of the film.

FIG. 5 shows the concentration profile, measured in the anodiccompartment for transport of K⁺ through the membrane. K⁺ ions show asimilar concentration profile to that of Na⁺ and again shows the twotransport rates, although insufficient data points were collected toclearly distinguish between these two rates.

Flux values indicate the quantity of ions (in mol) passing through a onecentimeter square portion of membrane every second. A flux value of0.5×10⁻⁸ mol /cm² sec for example, would indicate that, for a membranewith a surface area of 20 cm², every hour 14 mg of K⁺ ions aretransported through. The flux values for single ion transport (Na⁺, K⁺)are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Flux Values for K.sup.+  and Na.sup.+  Single Ion Transport                                K.sup.+ Na.sup.+                                                              (mol/cm.sup.2 s)                                                                      (mol/cm.sup.2 s)                                         ______________________________________                                        Initial Values 1 × 10.sup.-7                                                                     3.5 × 10.sup.-9                                Later Values   8 × 10.sup.-8                                                                       9 × 10.sup.-9                                ______________________________________                                    

When single ion transport experiments were carried out on Ca²⁺ however,almost no transport through the membrane was observed. After running asingle ion experiment on Ca²⁺, as for the single ion K⁺ experiments, 6ppm of Ca²⁺ was transported after 5 hours compared with 1200 ppm for K⁺.

FIG. 6 shows the concentration in the anodic and cathodic compartments,for the transport of an equimolar mixture of K⁺ (0.1M) and Na⁺ (0.1M)through the membrane. The calculated flux and selectivity factor values("s") for the mixed experiment (K⁺ /Na⁺) are given in Table 2. A similarexperiment to determine the transport of an equimolar mixture of K⁺(0.1M) and Ca²⁺ (0.1M) through a PPy membrane was also run and thecalculated flux and selectivity factor values are given in Table 3. Theflux rates are given in terms of the moles of ions transported persquare centimeter of polymer per second with the "s" value indicatingthe competition between the two.

                  TABLE 2                                                         ______________________________________                                        Flux and selectivity values for K.sup.+ /Na.sup.+  Transport System           Flux of K.sup.+ Flux of Na.sup.+                                                                        s values                                            (mol/cm.sup.2 s)                                                                              (mol/cm.sup.2 s)                                                                        (Flux K.sup.+ /Flux Na.sup.+)                       ______________________________________                                        Initial 4.5 × 10.sup.-9                                                                     3 × 10.sup.-9                                                                     1.5                                             Values                                                                        Later   3.5 × 10.sup.-10                                                                    1 × 10.sup.-9                                                                     3.5                                             Values                                                                        ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Flux and Selectivity values for K.sup.+ /Ca.sup.2+   transport system         Flux of K.sup.+ Flux of Ca.sup.2+                                                                       s values                                            (mol/cm.sup.2 s)                                                                              (mol/cm.sup.2 s)                                                                        (Flux K.sup.+ /Flux Ca.sup.2+)                      ______________________________________                                        Initial 5 × 10.sup.-8                                                                       3 × 10.sup.-8                                                                     1.6                                             Values                                                                        Later   4 × 10.sup.-9                                                                       2 × 10.sup.-9                                                                     2.0                                             Values                                                                        ______________________________________                                    

As can be seen in FIG. 6 the concentration profiles for the mixed iontransport (Na⁺ /K⁺) system measured in the anodic compartment, aresimilar in shape to those obtained for the single ion transport systemsand again show the two transport rates discussed previously.

A comparison of the flux values obtained (Tables 1 and 2) clearly showsthat competition occurs between the K⁺ and Na⁺ ions as the flux valuesobtained in the mixed ion systems (Table 2) are substantially lower thanthe single ion transport systems (Table 1).

Interestingly, in the mixed ion K⁺ /Ca²⁺ competitive system, significantquantities of Ca²⁺ were transported through the membrane (see Table 3).This was surprising, as (see previously) the single Ca²⁺ ion resultsindicated the transport of only minute quantities of Ca²⁺ ions. Thecooperative effect of transporting Ca²⁺ with K⁺, due to mixing ions, isnot clear. Further investigations are presently under way to investigatethis property however, that Ca²⁺ ions may be transported in this manneris clear from the results. This particular finding would be ofparticular interest to the wine industry as there is a need to removepotassium and calcium during wine production.

FIG. 7 shows the pH profiles for the mixed ion K⁺ /Na⁺ system. Thisindicates that charge neutrality is maintained during the transport ofions in one direction, by the migration of protons in the oppositedirection. On this basis, the observed change in the ion transport ratesobserved in FIGS. 4, 5 and 6 is probably due to the change in pH (ieproton depletion) in the anodic compartment. Simply by adding acid (eg0.1M HCl) to balance this proton depletion the rates of transportobserved will change from the dual rates (FIGS. 4, 5 and 6) to a lineartransport rate. Standard titre techniques as will be known in the artwill suffice to achieve this.

In order to determine the effect of allowing the potential through theconducting polymer membrane to vary a dual potentiostat was constructedwhich had the capacity to operate as either two independentpotentiostats or in a "Master/Slave" (M/S) mode. In the M/S mode, onecompartment was held at a constant potential (the "Master") and thepotential of the other compartment allowed to vary (the "Slave"). Thecurrent applied to the Slave compartment was equal and opposite to thatof the Master.

The concentration profile for the transport of K⁺ ions through themembrane, while operating in M/S mode, is shown in FIG. 8. As can beclearly seen the profile was linear with a flux value of 0.5×10⁻⁸mol/cm² sec. Thus monodirectional transport of ions can also be achievedin the M/S mode with a variation in potential across the membrane.

The electrochemical cell setups as disclosed in the present applicationtherefore provide a means of maximising the transport properties of themembrane and of inducing the mono-directional transport of ions througha conducting membrane. By applying a potential gradient across aconducting polymer membrane monodirectional transport of ions can beeffectively achieved.

The ability to switch conducting polymers between conductive andnon-conductive states leads to numerous applications in the area ofseparation technology. As mentioned previously the co-transport ofpotassium and calcium ions will be of particular use in the wineindustry. A further application is seen to be the removal of lithiumfrom geothermal bores. Such mono-directional transport removes theseions through the conducting polymer membrane from the wine/geothermalsolutions into a master/standard electrolyte by creation of thepotential gradient across the separating conducting polymer membrane.

In order to determine the selectivity and transport rates of mono-valentcations, the transport of Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ through a PPymembrane was studied. A solution comprising 5 ml of each of halide saltsof Li, Na, K, Rb and Cs (0.1M), was made up to 100 mL and placed in thecathodic compartment. The anodic compartment comprised CaCl₂ (0.1M, 100mL). As previously described, a potential gradient of +/-400 mV wasapplied and the rates of transport of the ions analysed by AA. Therelative rates of transport of the mono-valent cations are depicted inFIG. 9. As shown in FIG. 9 the relative selectivity of the membrane wasK⁺ (100%)≈Rb⁺ (98%)>Cs⁺ (80%)>Na⁺ (50%) Li⁺ (15%). The order ofselectivity is thought to reflect the selectivity of the sulphonatedopant ion for the various cations. The % uncertainty for the ions usedin the transport studies were: Li--3%, Na--1%, K--1%, Rb--2.5%, andCs--2.5%.

No real effect was observed on the transport rates of the fivemono-valent cations depicted in FIG. 9 when the potential gradient waschanged from +/-400 mV to +/-700 mV to +/-1000 mV.

The selectivity of the process for mono-valent, divalent and transitionmetal ions was also studied using solutions containing combinations ofthese ions. Solutions were prepared and used as the electrolyticsolution of the cathodic compartment. The membrane used was a PPy/PTSmembrane. A potential gradient was applied in the manner describedpreviously, and the rate of transport determined by the analysis ofsamples taken from the anionic compartment. The experimental conditionsfor the transport of the mono-valent ions across the PPy/PTS membrane isas has been described previously herein.

The experimental conditions for the transport of divalent cations acrossa PPy/PTS membrane are as follows. A solution comprising 5 ml of each ofthe halide salts of Li, K, Ca, Mg and Ba (0.1M), was made up to 100 mland placed in the cathodic compartment. The anodic compartment comprisedNaCl (0.1M, 100 ml). As previously described a potential gradient of+/-400 mV was applied and the rates of transport of the ions analysed byAA, the results of which are shown in Table 4. The results indicate thatmonovalent ions generally had a higher transport rate than divalent.

                  TABLE 4                                                         ______________________________________                                        The AA results (ppm) for the transport of mono and di-valent cations          across a PPy/PTS membrane.                                                    Time (sec)                                                                             0        9150   15040   75500 101800                                 ______________________________________                                         Li!     0.005    0.055  0.07    0.165 0.2                                     K!      1.22     5      5.55    9.2   10.05                                   Mg!     0.295    0.575  0.46    1.265 1.305                                   Ba!     0.02     0.334  0.475   1.6   2.6                                    ______________________________________                                    

The experimental conditions used to determine if transition metal ionscould be transported across the membrane is as follows. A solutioncomprising 5 ml of each of the halide salts of Li, K, Co, Cu and Ni(0.1M), was made up to 100 ml and placed in the cathodic compartment.The anodic compartment comprised NaCl (0.1M, 100 ml). A potentialgradient of +/-400 mV was applied and the anodic compartment analysed byAA. The results indicated that transition metals were unable to passthrough the membrane when the membrane has a constant potential gradientacross it.

To determine the applicability of the process in some typicalapplications, experiments were carried out using wine, geothermal borewater and sheep wool scour as the cathodic electrolytic solutions. Ineach case the analysis indicated the successful removal of cations fromthe solutions. Table 5 shows the flux rates obtained in the variousmedia.

FIG. 10 shows the transport of K⁺ ions from sheep wool scour. Thecalculated flux for K⁺ ion transport was approximately 1 mol hr⁻¹ m⁻²,and the energy consumption was in the order of 0.05 kW hr m⁻². Theprocedure for the removal of ions from the sample solutions is similarto that described earlier. A sample of the authentic industrial product(100 ml) was placed in the cathodic compartment and NaCl solution (0.1M,100 ml) was placed in the anodic. A potential gradient was applied, andsamples from the anodic compartment were analysed by AA.

                  TABLE 5                                                         ______________________________________                                        The Relative Flux Rate for Cations Being Removed From Wine,                   Geothermal Bore Water and Sheep Scour                                         Cathodic Compartment                                                                         Cations Fluxes/mol sec.sup.1 cm.sup.-2                         Electrolyte     K!        Na!      Li!                                        ______________________________________                                        Wine           3 × 10.sup.-9                                                                     --       --                                          Geothermal Bore Water                                                                        --        2 × 10.sup.-9                                                                    4 × 10.sup.-11                        Sheep Scour    5 × 10.sup.-8                                                                     --       --                                          ______________________________________                                    

As will be readily apparent to a person skilled in this particular artthe invention of the present application will have wide rangingapplicability in a variety of situations where there is a requirement toremove ions from solution. The experimental data disclosed in thisapplication have included wine, geothermal bore water, and sheep scourhowever the process should not be seen to be limited to suchapplications. Further alternatives may include, amongst many others, theremoval of lithium and/or radioactive caesium from sea water forexample.

The foregoing describes the invention including preferred forms thereof.It is to be understood that the scope of the invention is not to belimited to the specific forms described. Alterations and modificationsas will be obvious to those skilled in the art are intended to beincluded within the invention as described without departing from thespirit or scope of the invention as defined in the attached claims.

I claim:
 1. A method of inducing mono-directional transport of ionsbetween electrolyte solutions comprising separating the electrolytesolutions with a conducting polymer membrane and creating a potentialgradient across said membrane wherein the potential gradient is createdby using the conducting polymer membrane as a shared working electrode.2. The method of claim 1 wherein the potential gradient across theconducting polymer membrane is created via a three electrode system ineach electrolyte solution, said three electrode system comprising areference electrode, a counter electrode, and a shared workingelectrode, wherein the shared working electrode is the conductingpolymer membrane separating the electrolyte solutions.
 3. The method ofclaim 1 wherein the conducting polymer membrane is free-standing.
 4. Themethod of claim 1 wherein the conducting polymer membrane comprises acomposite membrane made of a conducting polymer incorporated into eithera non-conductive or conductive media.
 5. The method of claim 1 whereinthe conducting polymer membrane is selected form polypyrrol,polythiophene, polyaceteylene, polyfuran, polyaniline, or polyphenyleneor derivatives of these.
 6. The method of claim 1 wherein theelectrolyte solutions are constantly stirred.
 7. The method of claim 1wherein the potential gradient created across the conducting polymermembrane remains constant.
 8. The method of claim 1 wherein thepotential gradient created across the conducting polymer membrane isvaried.
 9. The method of claim 1 wherein the potential gradient createdacross the conducting polymer membrane is varied by holding oneelectrolyte solution at a constant potential and allowing the potentialin the other electrolyte solution to vary.
 10. The method of claim 1wherein there are more than two electrolyte solutions each separatedfrom each other by conducting polymer membrane and the potential in onesolution is held constant while the potential in each of the othersolutions is allowed individually to vary.
 11. The method of claim 1wherein there are more than two electrolyte solutions each of which areseparated from each other by the conducting polymer membrane.
 12. Themethod of claim 1 wherein the potential gradient across the conductingpolymer membrane is up to about 2400 mV.
 13. The method of claim 1 whenused to remove ions from wine, geothermal bore water, wool scour or seawater.
 14. The method of claim 1 wherein the ions transported throughthe conducting polymer membrane are cations or anions.
 15. The method ofclaim 1 wherein the ions transported through the conducting polymermembrane are cations and are selected from the group consisting of H,Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba and Ra.
 16. The method ofclaim 1 wherein the ions transported through the conducting polymermembrane are anions selected from the group consisting of the nitrates,sulphates, halides, phosphates and perchlorates.
 17. An apparatus forinducing mono-directional transport of ions between electrolytesolutions, said apparatus comprising container means for holding theelectrolyte solutions, a conducting polymer membrane separating theelectrolyte solutions and a three electrode system in each electrolytesolution, the three electrode system comprising a reference electrode, acounter electrode, and a shared working electrode and wherein the sharedworking electrode is the polymer membrane separating the electrolytesolutions.
 18. The apparatus according to claim 17 wherein theconducting polymer membrane is free-standing.
 19. The apparatusaccording to claim 17 wherein the polymer membrane is a compositemembrane made of a conducting polymer incorporated into either anon-conductive or conductive media.
 20. The apparatus according to claim17 wherein the conducting polymer membrane is selected from polypyrrol,polythiophene, polyaceteylene, polyfuran, polyaniline, or polyphenyleneor derivatives of these.
 21. The apparatus of claim 17 further includingcombination pH probes in each electrolyte solution.
 22. The apparatusaccording to claim 17 wherein the counter electrodes are stainlesssteel, titanium, gold, platinum or carbon counter electrodes.
 23. Theapparatus according to claim 17 wherein the reference electrode is anAg/AgCl or SCE electrode.
 24. The apparatus according to claim 17wherein said apparatus comprises more than 2 electrolyte solutions, eachsaid solutions being separated by a conducting polymer membrane and eachincluding said three electrode system.